The invention relates to a driver circuit for supplying a voltage having alternating polarities between a first and a second electrode of a flat panel display, and a flat panel display apparatus comprising a flat panel display and such a driver circuit.
Alternating voltages are required between electrodes of flat panel displays like LCD""s, Plasma Display Panels (PDP), Plasma Addressed Liquid Crystal displays (PALC), and Electro-Luminescent panels (EL). Due to a capacitance present between the electrodes, and required steep slopes of the alternating voltage, relatively large charge or discharge currents are required to reverse the polarity of the voltage across the capacitance. To minimize the power dissipation during the polarity reversion, driver circuits which comprise an energy recovery circuit in which an external inductance forms a resonant circuit with the capacitance are known from U.S. Pat. Nos. 5,081,400 and 5,670,974. Both these prior arts disclose an energy recovery circuit for a PDP.
A PDP may be driven in a sub-field mode wherein a plurality of successive sub-fields or frames occurs during a field or a frame of the video information to be displayed. A sub-field comprises an addressing phase and a sustaining phase. During the addressing phase, the plasma rows are selected one by one and data in conformance with the video information to be displayed is written into pixels of the selected row. During the sustaining phase, a number of sustain pulses is generated, dependent on the weight of the sub-field. Pixels pre-charged during the addressing phase to produce light during the sustaining phase will emit an amount of light during the sustaining phase which corresponds to the weight of the sub-field. The total amount of light produced by a pixel during the field or frame period of the video information depends, on the one hand, on weights of the sub-fields and, on the other hand, on those ones of the sub-fields during which the pixel was pre-charged to produce light.
In a PDP, the two electrodes may be the scan electrodes and the common electrodes. Cooperating scan electrodes and common electrodes form pairs which are each associated with one of the plasma channels. During the sustaining phase, the pairs of electrodes are driven with anti-phase square-wave voltages generated by a full-bridge circuit The full-bridge circuit comprises a first series arrangement of a first and a second controllable switch, and a second series arrangement of a third and a fourth controllable switch. A junction of main current paths of the first and the second switch is coupled to a scan electrode. A junction of main current paths of the third and the fourth switch is coupled to a common electrode. The first series arrangement and the second series arrangement are arranged in parallel across terminals of a power supply source. The main current path of the first switch is arranged between the scan electrode and a first one of said terminals, the main current path of the third switch is arranged between the common electrode and said first terminal. During a first phase of a sustaining period, two of the switches are open, whereas two of the other switches are closed, such that the power supply voltage supplied by the power supply source is available in a first polarity between the cooperating electrodes and thus across the capacitance. During a second phase of the sustaining period, the switches which were open during the first phase are now closed, and the switches which were closed are now open, such that the power supply voltage supplied by the power supply source is available in the reversed polarity between the cooperating electrodes.
U.S. Pat. No. 5,081,400 uses a large capacitor to store the recovered energy. U.S. Pat. No. 5,670,974 does not require such an extra energy storage capacitor. Both prior arts require further controllable switches in addition to the controllable switches of the full bridge.
It is, inter alia, an object of the invention to provide a driver circuit for a flat panel display, which driver circuit comprises a less complex energy recovery circuit.
To this end, a first aspect of the invention provides a driver circuit as claimed in claim 1. A second aspect of the invention provides a flat panel display apparatus as claimed in claim 8. Advantageous embodiments are defined in the dependent claims.
The driver circuit in accordance with the invention is able to provide energy recovery by adding, in series with the capacitance, a series arrangement of an inductor and a diode. The series arrangement of the capacitance, the inductor and the diode is arranged in parallel with the first switch of the full bridge. The diode is poled to be non-conductive during the first phase and the second phase wherein the four switches of the bridge are controlled by the control circuit to be on and off, such that the power supply voltage is available across the capacitance in the first and in the reversed polarity, respectively. The diode is conductive during a third phase which occurs in between the first and the second phase. In this third phase, wherein the control circuit closes the first switch, the series arrangement of the capacitance, the inductor and the diode forms a resonant circuit, and the voltage across the capacitance will change polarity in an energy-efficient way. The transfer from the first to the second phase is performed via the energy recovery during the third phase, only by controlling the already present switches of the full bridge. No additional controllable switches are required.
In an embodiment as defined in claim 2, a further series arrangement of an inductor and a diode is added to form a series arrangement of the capacitance, the inductor and the diode which is arranged in parallel with the third switch of the full bridge. Now, a fourth phase occuring after the second phase. In this fourth phase, the inductor of this further series arrangement forms a resonant circuit with the capacitance to allow an energy-efficient transfer from the reversed polarity to the first polarity of the voltage across the capacitance. Thus, when this embodiment of the invention is applied during a sustain period of a PDP, positive and negative voltage pulses are successively applied between cooperating scan and common electrodes. The energy recovery during the transition periods when the pulses change sign is obtained by controlling the switches of the full bridge in such a that way during these transition periods, the first-mentioned or the further series arrangements of the inductor and the diode form a resonant circuit with the capacitance.
In an embodiment as defined in claim 3, the second and the fourth controllable switches comprise an internal anti-parallel diode. For example, MOS transistors are controllable switches which have such internal diodes. The third and fourth diodes allow a negative voltage at the first and second electrodes.
In an embodiment as defined in claim 4, the third and fourth diodes allow a voltage at the first and second electrodes to have an absolute value exceeding the absolute value of the voltage supplied by the power supply source.
In an embodiment as defined in claim 5, parasitic currents are minimized. For example, a parasitic current will flow through a drain source capacitor of the fourth switch when the third switch is closed at the start of the resonance period. This current, which is supplied by a first terminal of the second capacitor, will flow via the fifth and the sixth inductor to the second junction which is the other terminal of the second capacitor. The series arrangement of the fifth and the sixth inductor forms a high impedance for this parasitic current. The main currents, which flow during the first and second phase and which are the plasma currents in a PDP, will not flow through the series arrangement of the fifth and the sixth inductor and will consequently not be negatively influenced by the presence of these inductors. A more detailed description of this feature is given with respect to FIG. 4.
In an embodiment as defined in claim 6, it is possible to supply a negative voltage to the electrode to which the series arrangement of the diode and the inductor is connected. If the series arrangement of the diode and the inductor is arranged between the capacitance and the negative terminal of the power supply source, a negative voltage on the electrode would be counteracted by the diode which would conduct.
In an embodiment as defined in claim 7, only a single inductor is required, but without additional components it is not possible to supply a negative voltage to the electrode to which the diodes are coupled via the inductor.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.